Fuel burner unit for mounting in a common air box

ABSTRACT

A FUEL BURNER UNIT DESIGNED TO BE CONNECTED TO A COMMON AIR SUPPLY BOX IN PARALLEL WITH ONE OR MORE LIKE BURNER UNITS. THE BURNER COMPRISES A DUCT SYSTEM FOR CONVEYING COMBUSTION AIR FROM THE AIR BOX TO THE BURNER HEAD AND INCLUDES A SINGLE ANNULAR OUTER DUCT CONNECTED AT ITS FRONT END TO THE AIR BOX AND AT ITS REAR END TO INDIVIDUAL INNER DUCTS FOR PRIMARY AND SECONDARY COMBUSTION AIR, RESPECTIVELY. THE VOLUME RATIO BETWEEN PRIMARY AND SECONDARY AIR IS DETERMINED BY AN ADJUSTABLE, BUT NORMALLY FIXED BAFFLE DEVICE AT THE REAR OUTLET END OF THE OUTER DUCT, THE TOTAL AMOUNT OF COMBUSTION AIR SUPPLIED TO EACH BURNER IS CONTROLLED BY A VALVE DEVICE LOCATED AT THE INLET END OF THE ANNULAR OUTER DUCT.

June 27, 1972 E. BENDIXEN FUEL BURNER UNIT FOR MOUNTING IN A COMMON AIRBOX Filed NOV. 30. 1970 2 Sheets-Sheet 1 June 1972 K. E. BENDIXEN 3,

FUEL BURNER UNIT FOR MOUNTING IN A COMMON AIR BOX Filed Nov. 30. 1970 2Sheets-Sheet 2 United States Patent Office 3,672,812 Patented June 27,1972 Int. (:1. F23n US. Cl. 431-89 7 Claims ABSTRACT OF THE DISCLOSURE Afuel burner unit designed to be connected to a common air supply box inparallel with one or more like burner units. The burner comprises a ductsystem for conveying combustion air from the air box to the burner headand includes a single annular outer duct connected at its front end tothe air box and at its rear end to individual inner ducts for primaryand secondary combustion air, respectively. The volume ratio betweenprimary and secondary air is determined by an adjustable, but normallyfixed bafile device at the rear outlet end of the outer duct. The totalamount of combustion air supplied to each burner is controlled by avalve device located at the inlet end of the annular outer duct.

BACKGROUND OF THE INVENTION The present invention relates to a fuelburner unit for boilers, furnaces and other heat exchanging apparatus,and of the type which is intended to be mounted in an air box common toseveral units and which has at least two parallel flow ducts for thesupply of primary and secondary combustion air. It is well-known that analmost completely or essentially stoichiometric combustion isadvantageous by yielding flue or exhaust gases with a low sulphurtrioxide content and, thereby, permits low smoke or flue temperatureswithout risk of corrosion damages. However,

stoichiometric combustion requires a precise matching of the air volumesupplied to each burner unit in dependence upon the amount of fuelsupplied, which, to date, has proved difficult to effect in actualpractice. Boilers have been constructed having a completely separate andrelatively long air supply duct for each burner, i.e. without a commonair box, but this represents a complicated and expensive solution from aconstructional point of view and, moreover, presents difficulties withrespect to the desired automatic control of the firing plant. In plantshaving a common air box it is known to effect a separate control of thevolumes of primary and secondary air supplied to the individual burners,but this complicates the control and, consequently, in other knowndesigns, the entire volume of combustion air has been suppliedcollectively to each burner without any division into primary andsecondary air being effected, while various designs of the burner andthe air duct have been employed in an attempt to obtain an acceptablecourse of the combustion process. However, experience gained from actualpractice shows that the chances of optimizing the combustion process bymeans of this solution are limited.

BRIEF SUMMARY OF THE INVENTION According to the invention there isprovided a fuel burner unit comprising an open front end and a closedrear end, duct means for supplying fuel to said burner front end, atleast two parallel duct means for supplying primary and secondarycombustion air, respectively, to said burner front end, an annular ductsurrounding said two duct means and connected thereto at its rear end,

fixed air distributor means for said primary and secondary air ductmeans located at the rear end of said annular duct, means at the frontend of said annular duct for connecting the duct to an air box common toa plurality of burner units, adjustable air valve means at said frontend of said annular duct, means for measuring the air flow rate throughsaid annular duct, and means for adjusting said air valve means independence upon the measured air flow rate.

It has surprisingly been found that the invention provides asatisfactory solution of the problems pointed out above. The supply ofthe total air volume to each burner via a common duct and the control ofthe air volume with the aid of a valving means located at the inlet ofthe duct where it is connected to the common air box provides astructurally simple design of the burner and the associated controlsystem; it has also been found possible to ensurewithin a wide loadrangea satisfactory distribution of the total air volume into theprimary and secondary air system, respectively, without any change inthe setting of the distributor provided at the rear end of the burner.Consequently, an adjustment of this setting is required only when thefiring plant is taken into service or when there are material changes inthe operating conditions, for instance, when changing over to adifferent fuel. When, according to the invention, primary and secondaryair is supplied separately to the burner head or front end, while thevolumetric control is effected on the total air volume supplied to theunit, it is possible to optimize the combustion process and generally toobtain those advantages which stem from dividing the air into primaryand secondary air streams and from using a common air supply,respectively, without the respective concomitant drawbacks to which theknown designs were subject.

The flow rate measuring means may be adapted to measure the differencebetween the static pressure at the valving means and at a locationintermediate the ends of the duct, respectively, as this pressuredifference depends on the air velocity in the duct and, hence, on theair flow rate.

According to a feature of the invention, there may be provided ameasuring chamber connected to the measuring means, whichchambercommunicates with the annular duct but is otherwise closed,adjacent and downstream of the valving means at the front end of theannular duct. In this way, a particularly accurate value of the staticpressure at the inlet of the duct is obtained independently of thepressure conditions prevailing in the air box and the possible pressuredrop across the valving means.

The annular duct may be designed as a Venturi nozzle, and the staticpressure intermediate the ends of the duct may be measured at thenarrowest cross section of the nozzle, whereby a certain recovery of thepressure drop in the constriction of the nozzle is ensured whichfavourably affects the economy of operation of the burner.

If the available space does not permit the relatively great projectinglength of the burner unit which the Venturi shape requires, an orificerestrictor or diaphragm may alternatively be mounted in the annular ductand the measuring means may be connected on the upstream and thedownstream side of the orifice restrictor, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a longitudinal sectionthrough an embodiment of the burner unit according to the inventionmounted in a common air box,

FIGS. 2 and 3 show two corresponding longitudinal sections throughmodified embodiments of the burner unit,

FIG. 4 shows a cross section along the line 44 in FIG. 1, and

FIG. 5, shows a schematic damper control.

3 DETAILED DESCRIPTION All the burner units shown in the drawings aredesigned for being mounted in an air box 7 common to several burnerunits, which box extends along a combustion chamber 15 (not shown indetail) and is provided with means (not shown) for supplying combustionair in the requisite amount thereto.

Each burner unit comprises a centrally located supply duct or line 16for the fuel which may be of any type, liquid, gaseous or pulverizedsolid fuel. In the front end of the fuel line 16, that is to say at theleft-hand side of FIGS. 1, 2 and 3, there may be provided means (notshown) for distributing and/or atomizing the fuel into the combustionchamber 15. Such means may be of any known type.

In all the embodiments shown, the air necessary for the combustion ofthe fuel is supplied to the burner head or front end' via two separateducts, viz a central duct 1 surrounding the supply line 16 and throughwhich primary air is supplied, and a secondary air duct 2 surroundingthe duct 1 coaxially. At the front end of each of the ducts 1 and 2,so-called turbulators 3 and 4, respectively, are schematically shown,which turbulators may consist of inclined or curved plates which imparta suitable rotation to the two air streams before they enter thecombustion chamber through a primary air nozzle 5 and a secondary airnozzle 6, respectively. The tubulators 3 and 4 may be fixed or they maybe adjustable with the aid of moving means (not shown).

In the embodiment of FIGS. 1 and 4, an annular flow duct 9 is definedbetween the external wall of the burner unit and the secondary air duct2, and the front end of the flow duct 9 communicates with the air box 7via a conically constricted inlet 8. At the rear end of the duct 9, anair distribution means in the form of an annular baifie device 10 ismounted comprising a central sleeve 17, which is guided on the wall ofduct 1, and an annular baffie plate 18, the outermost edge of which iscurved so as to extend into the discharge or rear end of duct 9. Thebafile device 10 divides the air stream coming from the duct 9 into aprimary air stream which continues through the internal duct 1, and asecondary air stream flowing through duct 2, respectively. The sleeve 17may be displaceable in its longitudinal direction along the wall of theduct 1, so that it is possible to vary the ratio between thecross-sectional areas through which the air flows from duct 9 into eachof the ducts 1 and 2, and hence, the mutual ratio between the volumes ofprimary and secondary air. However, during the operation of the burner,the battle device 10 may remain stationary since an adjustment of thesaid ratio normally only is required when there are significant changesin the operational conditions.

Duct 9 including the inlet 8 is shaped as a Venturi nozzle, and in theconstricted section thereof there is an aperture 19 in the outer wall ofthe duct for connecting a schematically indicated differential pressuregauge 20. The other measuring point of gauge 20 is constituted by asimilar aperture 21 in the peripheral wall of a measuring chamber 13which wall is closed except for the aperture 21 and is coaxial with theducts 1, 2 and 9. The chamber 13 is located closely adjacent anddownstream of an annular damper or valve 14 which is mounted in the airbox 7 coaxially with the burner unit and which may be displaced axiallyto vary the inflow cross section area of the duct inlet 8. The measuringchamber 13 communicates, via one or more apertures 22 in its end wall,with the inlet 8.

As shown in FIG. 1, the pressure gauge 20 senses the difference betweenthe static pressure prevailing in the measuring chamber 13 and, hence,immediately downstream of the damper 14, and the pressure at thenarrowest section of the duct 9. Consequently, the differential pressuremeasured will indicate the total air volume passing through duct 9 perunit of time since the cross sectional area at the measuring point 19 isknown. The said differential pressure which, in FIG. 1, is indicatedschematically in the form of a liquid column Ap, may in a manner knownper se be utilized as input signal in a control system (see FIG. 5) forcontinuously adjusting the damper 14 in such a way that the total volumeof air supplied to each burner is kept at a predetermined value which,in turn, depends on the amount of fuel supplied via the line 16. With asingle measuring device for each burner unit and a correspondinglysimple control system, it is thus possible, independently of anypressure variations in the air box, to control the air volumes suppliedto the individual burners of a firing plant in a desired manner and insuch a way that optimal conditions for combustion in the individualburner, units are obtained. During the running-in period of the firingplant, it is possible, for each individual burner unit, to set or adjustexperimentally the transfer function between the measuring signalderived from the differential pressure gauge 20 and the driving means(not shown) which displace the damper 14 to vary the inflow area of duct9 from the air box 7. In an analogous manner, it is possible toexperimentally effect the adjustment of baffie device 10 whichadjustment may normally be maintained in the course of the subsequentoperation of the unit.

The embodiment illustrated in FIG. 2 corresponds, so far as importantelements are concerned, to that shown in FIG. 1, and for these elements,the same reference numerals are employed as in FIG. 1.

The difference resides in the constructional design of the common airsupply duct surrounding the primary and secondary air ducts 1 and 2.While the duct in FIG. 1

was constructed in the form of a Venturi nozzle and,

consequently, was relatively long, so that the air could expanddownstream of the narrowest cross section of the nozzle, thecorresponding duct 11 in FIG. 2 is considerably shorter. The reductionin the space requirement of the burner unit achieved hereby isaccompanied by a minor decrease in the economy of operation due to theincreased energy loss on account of the higher air velocity of the duct11 and in the transition area to the ducts 1 and 2 including the bafiledevice 10. The mode of operation is, however, generally the same as forthe embodiment in FIG. 1. a

FIG. 3 shows an embodiment which, like that shown in FIG. 2, isdistinguished by a short structural length of the complete burner unit,but in which the total air volume supplied is measured by measuring thedilferental pressure across an orifice restrictor 12 or apertureddiaphragm located in the annular duct between the inlet 8 and the bafiledevice 10. In FIG. 3, the measuring points of the differential pressuregauge 20 are located on the downstream side of the orifice restrictor 12and in the measuring chamber 13 described above, respectively, but thelatter measuring point might also be located directly on the upstreamside of the orifice restrictor in the outer wall of the annular airduct.

I claim:

1. A fuel burner unit comprising an open front end and a closed rearend, duct means for supplying fuel to said burner front end, at leasttwo parallel duct means for supplying primary and secondary combustionair, respectively, to said burner front end, an annular enclosuresurrounding said two duct means, air distributor means located at therear end of said annular enclosure to divide an incoming stream for saidprimary and secondary air duct means, means at the front end of saidannular enclosure for connecting it to an air box, adjustable air valvemeans at said front end of said annular enclosure, means for measuringthe air flow rate through said annular enclosure, and means foradjusting said air valve means in dependence upon the measured air fiowrate.

2. A fuel burner unit as claimed in claim 1, wherein said flow ratemeasuring means comprises means for sensing the difference between thestatic air pressure in the region of said air valve means and at alocation intermediate the front and rear ends of said annular enclosure.

3. A fuel burner unit as claimed in claim 2, comprising means defining ameasuring chamber located adjacent and downstream said air valve meansand communicating with said annular enclosure and with said flow ratemeasuring means.

4. A fuel burner as claimed in claim 3, wherein said measuring chamberis annular and comprises a closed peripheral wall coaxially surroundingsaid duct means and having an apertured end wall facing said rear end.

5. A fuel burner unit as claimed in claim 2, wherein said annularenclosure is formed as a Venturi nozzle, and said intermediate locationin which the static pressure is measured, is at the narrowestcross-section of said nozzle.

6. A fuel burner unit as claimed in claim 2, comprising an apertureddiaphragm located intermediate the ends of said annular enclosure, saidintermediate location being situated at the downstream side of saiddiaphragm.

7. A fuel burner unit as claimed in claim 1, wherein said air valvemeans is in the form of an open-ended drum arranged coaxially with saidannular enclosure and adjustable in the longitudinal direction thereof.

References Cited UNITED STATES PATENTS CARROLL B. DORITY, 111., PrimaryExaminer US. Cl. X.R.

